Attended one of his lectures many years ago and he was indeed using Emacs back then. Don't remember which OS but I think the whole session he was just in Emacs most of the time. Also one time he stopped and was puzzled what was going on, quickly realizing that he'd forgotten to save the file. Still to this day when I forget to save I remember this and think : "If it can happen to Knuth it's ok if it happens to you"